The present invention relates to a tunneling Josephson junction of oxide superconductor and a fabrication method thereof.
A conventionally known tunneling Josephson junction uses as its superconducting material a Pb-In-Au alloy, a Pb-In-Sn alloy or a Pb-In alloy containing Pb as its main component. A Josephson junction is obtained by interposing a thin barrier layer of 20 to 30 A thickness between two superconducting materials and weakly coupling them. Such a barrier layer is prepared by forming an ultra thin oxide layer on the surface of an alloy which is a superconducting material by thermal oxidation for oxidation in plasma discharge. For example, in the case of the Pb-In-Au alloy, a layer of In.sub.2 O.sub.3 or PbO (PbO.sub.x) is formed as a barrier layer on its surface.
The barrier layer and the superconducting materials are totally different crystallographically and have different thermal expansion coefficients. Due to this, the service life under heat cycles between room temperature and ultra low temperatures is about 500 cycles at maximum. After this limit, the oxide layer as the barrier layer breaks down.
Breakdown of the oxide layer as the barrier layer is related to the fact that the superconducting materials are soft metals whereas the oxide layer is hard. It follows, therefore, that breakdown of the barrier layer may be prevented if the superconducting materials are oxides as in the case of the barrier layer. As a prior art technique which uses oxides as the superconducting materials, U.S. Pat. No. 3,932,315 may be mentioned. This patent specification discloses that "compositions of the formula Ba.sub.1-x A.sub.x Pb.sub.1-y Bi.sub.y O.sub.3 are superconductors" but does not disclose any method for fabricating a tunneling Josephson junction, above all, a Josephson junction having a structure which realizes fine patterning and thinner film.
When an oxide superconducting material is composed of several kinds of elements, it is generally difficult to form a thin film by the conventional evaporation method. With the sputtering method under usual conditions, the surface becomes coarse, the film is amorphous, and a thin film of good quality is not obtainable. When this amorphous film is annealed at a temperature above the crystallization temperature, most of the elements constituting the thin film tend to evaporate. When Pb is included in these elements, it tends to form other lead oxides or react with the substrate so that a desired oxide superconducting electrode layer may only be formed in a small amount. Thus, with the conventional methods, the fabrication of thin films of oxide superconducting materials is still in the experimental stage, and thin films of good quality requiring fine structures for Josephson junctions have not been available.